Method for design respective manufacture of threads in a threaded member, a threaded member manufactured according to the method, and a bolted joint including such a member

ABSTRACT

The invention relates to a method in the formation of threads of a thread-provided element intended to be included as one of two thread-provided parts in a bolted joint. The bolted joint comprises a part ( 2 ) having an internal thread and a part ( 1 ) having an external thread, and the threads of the respective parts are initially formed according to the ISO and/or Unified basic profile having an initial play in both the radial and axial direction, when said parts are tightened together. The invention is characterized in that the threads of said thread-provided element are formed so that an additional play (ΔS a ) between said two thread-provided parts is obtained in the axial direction, at the same time as said initial play in radial direction remains substantially unaltered, the part of the bolted joint which consists of the thread-provided element no longer having ISO/Unified basic profile. The invention also relates to a method for the manufacture of threads, as well as corresponding thread-provided element and bolted joint.

The present invention relates to a method in the formation of threads of a thread-provided element intended to be included as one of two thread-provided parts in a bolted joint, which bolted joint comprises a part having an internal thread and a part having an external thread, and where the threads of the respective parts initially are formed according to the ISO and/or Unified basic profile having an initial play in both the radial and axial direction, when said parts are tightened together. The invention also relates to a thread-provided element formed according to the method as well as a bolted joint where at least one of the thread-provided parts of the joint consists of such a thread-provided element.

The invention furthermore relates to a method for the manufacture of threads of a thread-provided element intended to be included as one of two thread-provided parts of a bolted joint, a thread-provided element manufactured according to the method as well as a bolted joint where at least one of the thread-provided parts of the joint consists of such a thread-provided element.

Thus, the thread-provided element in question according to the invention may be either a screw or a nut, and said screws or nuts provided with threads formed according to the invention may either be used together with each other or together with screws and nuts formed according to standard.

According to the ISO standard (ISO/68) for metric threads, as well as for threads according to the American Unified standard (ANSI/ASME B1.1 which refers to ISO/68), a so-called basic profile is used which is illustrated in FIG. 1. According to this basic profile, the width, among other things, is defined by the thread ridge of the screw and the thread grooves of the nut, respectively, as half the pitch (0,5P), and the theoretical mean diameter for screw and nut, respectively, d₂ and D₂, respectively, is determined by the generatrix that intersects the basic triangle on half the height H thereof. In the following, additionally a number of concepts and expressions are used according to the definitions that are given in ISO/68.

Theoretically, the mean diameters for screw and nut, respectively, coincide with each other according to the basic profile. However, in practice, there has to be a play between screw and nut in order to be assemblable.

By using standardized manufacturing tolerances, a play is obtained which gives replaceability between screw and nut from different manufacturers. The play is provided by displacing the mean diameter of the screw inwards, or the mean diameter of the nut outwards. Thereby, both a radial and an axial play is obtained, see FIG. 2. However, thereby also the thread coverage, i.e. the distance which the threads of the screw and of the nut overlap each other in radial direction, decreases correspondingly, which makes the surface pressure between the threads increasing, and the strength deteriorating.

According to Hooke's law, a body is extended proportional to the force that is applied on it. Such an extension may also be brought about by heating. To a screw, an extension means that the thread pitch increases. If the extension of the screw within the height of the nut becomes greater than the axial play between the flanks of the threads, interference will arise, which involves that the nut get locked.

This problem is particularly noticeably at high prestressing forces, as is the case at hydraulic prestress of high-strength screw in dimension M18 and upwards, with thread tolerances according to standard, and nut greater height than standard. During the prestress, the nut is, in the end of the screw where the prestressing tool pulls, unloaded while the prestressing tool pulls the screw with an axial tensile force. High-strength screw is characterized by the force obtained at prestress near the yield point in tension being significantly greater than for ordinary screws, and that it thereby requires more threads to distribute the force among so that the stress per thread does not become too high, which results in a greater nut height than standard, e.g. of the order of 100%. The axial extension of the screw that is obtained within the greater nut height has, however, still to be housed within the axial play of the nut in order to avoid interference. Thus, use of high-strength screw implies an increased risk of locking when thread tolerances according to standard do not give enough axial play. Thus, the size of the play sets a limit for how much the screw may be prestressed. In FIG. 3 is illustrated schematically how the threads of a prestressed screw co-operates with the threads of a screwed-on nut and the axial play which is required for locking not to occur.

The prevalent method to bring about an axial play is, according to the above, to give the screw a smaller mean diameter than the nut. It is, however, inappropriate, and occasionally even impossible, to additionally increase the difference in mean diameter between screw and nut so as to thereby obtain a sufficient axial play, since this decreases the thread coverage and implies higher surface pressure between the threads, as has been explained above.

The object of the invention is to overcome the above-mentioned disadvantages and find a solution to the problems mentioned above. This takes place by the method for the formation of the threads of a thread-provided element, such as a screw or a nut, which has the new features defined in the characterizing part of claim 1, as well as the method for the manufacture of threads of a thread-provided element intended to be included as one of two thread-provided parts in a bolted joint, which has the new features defined in claim 11.

Thus, the method for the formation of the threads, according to the invention, is characterized in that the threads of the thread-provided element are formed so that an additional play between said two thread-provided parts is obtained in the axial direction, at the same time as said initial play in radial direction remains substantially unaltered, the part of the bolted joint which consists of the thread-provided element no longer having ISO/Unified basic profile. In this way, the advantage is obtained that the thread coverage may be maintained and the largest feasible abutment between the two parts is obtained, which gives an opportunity to a maximum transfer of forces. This enables a high prestressing force of a screw, which is particularly advantageous for screws in high-strength material. However, the invention is not limited to the application for high-strength screw but may advantageously be used also in other applications.

According to an advantageous embodiment, the invention is characterized in that the part having an external thread is intended to be prestressed, that the thread-provided element consists of the part having an internal thread the threads of which are formed so that said additional axial play together with the initial axial play equals at least the maximal elongation which the part having external thread is expected undergo, at the prestress, within the area where said parts co-operates.

According to another advantageous embodiment, the invention is characterized in that the thread-provided element consists of the part having external thread and that said part is intended to be prestressed, that the threads of the part having external thread is formed so that said additional axial play together with the initial axial play corresponds at least the maximal elongation which the part having external thread is expected undergo, at the prestress, within the area where said part having external thread co-operates with the part having internal thread.

According to an advantageous aspect, the width of the thread ridge of the thread-provided element is decreased by a measure corresponding to the desired additional axial play. This may be effected either by the decrease of the width of the thread ridge only occurs on one flank thereof, or is distributed on the two flanks thereof.

Furthermore, the present invention provides a thread-provided element formed according to the method of formation, as well as a bolted joint comprising at least one such thread-provided element.

The method in the manufacture of threads, according to the present invention, is characterized in that the threads are manufactured in two step, viz. a first step wherein the threads are manufactured by means of a cutting tool in the same way as in the manufacture of threads according to the ISO standard or Unified standard with the appurtenant initial axial and radial play, and a second step where the cutting tool only is advanced axially an additional distance in order to bring about an additional axial play decided beforehand, which distance equals said play, as well as with an unaltered radial position. The advantages that are attained have already been described previously, but additional advantages are obtained by elements having the new thread of the invention that gives additional axial play simply may be manufactured starting from the manufacturing method applying to elements having usual standard threads, with the same equipment and with a very small changing in the control. The two steps may follow directly one after the other, non-stop. The invention also relates to a thread-provided element manufactured according to said method, as well as a bolted joint comprising at least one such thread-provided element.

The invention will now be disclosed in detail reference being made to the accompanying drawing figures, which illustrate prior art and the problems therewith, as well as show embodiment example of the present invention, and in which:

FIG. 1 shows the basic profile for threads according to the ISO standard and Unified standard,

FIG. 2 illustrates schematically how an extended axial play between screw and nut is achieved according to the ISO standard, in a part view from the side and in cross-section of a bolted joint,

FIG. 3 illustrates schematically how prestress of a screw affects the play between screw and nut in a bolted joint, seen in a part view from the side and in cross-section,

FIG. 4 shows a part view from the side and in cross-section of an embodiment example of a bolted joint having threads formed according to the present invention, and

FIG. 5 illustrates the manufacturing method according to the present invention.

As already has been mentioned in the introduction of the description, FIG. 1 shows the basic profile for threads, which is common to ISO/68 and Unified standard. The screw 1 is here the part that is under the basic profile line and the nut 2 is the part that is above the basic profile line in the figure. P is the thread pitch, H is the basic triangle height for screw and nut, respectively, d₂ is mean diameter for screw and D₂ is mean diameter for nut.

In FIG. 2 is shown schematically how a play between screw 1 (the upper part) and nut 2 (the lower part) according to the ISO standard and Unified standard is achieved according to prevalent practice, i.e. by displacement of the mean diameter d₂ of the screw in relation to the mean diameter D₂ of the nut, which also has been accounted for in the introduction of the description. Hereby, also the thread coverage decreases from having been GT_(g) according to the basic profile to becoming GT₁. In radial direction, a play S_(r) is obtained and in the axial direction a play S_(a) is obtained, which in the illustrated case is distributed on the two flanks of the thread so that the axial play at the respective flank is S_(a)/2.

In FIG. 3 is illustrated schematically how the threads of a prestressed screw 1 co-operate with the threads of a screwed-on nut 2 and the axial play that is required so that locking will not occur. To the left in the illustration is seen how, at one end of the nut, almost total abutment against the right flank 3 of the screw thread takes place, i.e. no play at said flank but all axial play 4 is located to the opposite flank, while at the other end of the nut, to the right in the illustration, abutment takes place against the left flank 5 of the screw thread. In the central portion of the nut, the axial play 4 is evenly distributed on both flanks.

In FIG. 4, an embodiment example is shown of formation of threads according to the present invention. An increased total axial play S_(a)′, distributed with half on each of the respective flanks and which also can be described as the initial axial play S_(a) with addition of an additional axial play ΔS_(a), has been obtained in comparison with FIG. 2. This has been obtained by the fact that the mean diameter D₂′ for the nut 2 has been moved out the distance α, i.e. the distance between mean diameter D₂′ of the nut and the mean diameter d₂ of the screw 1 has increased. This has, however, been allowed to occur without the radial play S_(r) being changed. Thus, the thread coverage has been maintained and is still equally large as the thread coverage in FIG. 2, i.e. GT₁. Had instead an increased axial play been provided according to the prevalent method, as is illustrated in FIG. 2, the thread coverage would have decreased in a way that corresponds to the one that is seen in FIG. 2, where GT₁ is smaller than GT_(g). As also is seen in FIG. 4, the screw has maintained the basic profile thereof according to ISO/Unified standard while the modified nut, however, no longer complies with this basic profile.

Finally, an embodiment example is shown in FIG. 5 of the method in the manufacture of threads of a thread-provided element intended to be included as one of two thread-provided parts in a bolted joint, according to the present invention. The bolted joint comprises a part having an internal thread in the form of a nut 2 and a part having an external thread in the form of a screw 1. As an example, a thread-provided element in the form of a nut 2 is shown in FIG. 5. The threads are manufactured in two steps, viz. a first step wherein the threads are manufactured by means of a cutting tool in the same way as in the manufacture of threads according to the ISO standard or Unified standard with the appurtenant initially axial and radial play, and a second step where the cutting tool only is advanced axially an additional distance so that the width of the thread ridge is decreased by a measure corresponding to the desired additional axial play ΔS_(a). This second step with axial feeding takes place with unaltered radial position. In FIG. 5 is illustrated how the decrease of the thread ridge takes place by one flank thereof being cut down. The position of the flank after the first step of machining is illustrated by the dashed line 8, and the position of the flank after the second step of machining is illustrated by the continuous line 9. Thus, all machining only occurs on one of the flanks of the thread ridge, but it is also feasible to machine both the flanks, if this is considered convenient. It is important to note that the radial position of the truncated top surface 10 is not effected by the machining in step two, the radial play remains thus unaltered and the thread coverage is maintained.

It should be added that in all illustrated cases, the bottoms of the threads are somewhat rounded, according to standard, but this has not has been shown in the drawings for reasons of clarity.

In the shown examples, the nut has been illustrated as the part that has been formed with a thread that gives an additional play according to the present invention, but it may of course as well be the screw that is formed in this way, or alternatively both parts, if this is considered convenient.

The present invention should not be regarded as limited by the embodiments which have been disclosed, in terms of examples only, but may be modified and varied in a of multiple of ways, as is realized by a person skilled in the art, within the scope of the accompanying claims. 

1. Method for forming threads of a thread-provided part which is intended to be one of two thread-provided parts in a bolted joint, the method comprising: providing a first part having an internal thread and a second part having an external thread wherein the first and second parts are screwed together with the second part is intended to be inside the first part, and wherein the second part having the external thread intended to be prestressed with an axial tensile force, the threads of the respective parts initially being formed according to a basic profile which provides an initial play in both radial and axial directions, when the parts are screwed together and are unstressed, further forming the threads of at least one of the thread-provided parts such that an additional play ΔS_(a) between the two thread-provided parts is obtained in the axial direction, while the initial play in the radial direction remains substantially unaltered, and the at least one of parts of the bolted joint which is formed to provide the additional play, no longer having the basic profile.
 2. Method according to claim 1, wherein the at least one thread-provided part is the first part having the internal thread which includes threads formed so that the additional axial play together with the initial axial play corresponds to at least a maximum elongation which the second part having the external thread is expected to undergo, at the prestress, within an area where the parts cooperate.
 3. Method according to claim 1, wherein the at least one thread-provided part is the second part having the external thread, and the external thread being formed so that the additional axial play together with the initial axial play corresponds to at least a maximum elongation which the second part having the external thread is expected to undergo, at the prestress, within an area where the second part having the external thread co-operates with the first part having the internal thread.
 4. Method according to claim 1, further comprising increasing the axial play with maintained thread coverage.
 5. Method according to claim 1, wherein the threads of the parts have respective thread ridges, of an initial width, the method further comprising decreasing the width of the thread ridge of the at least one thread-provided part by a measure corresponding to the desired additional axial play.
 6. Method according to claim 5, wherein the thread ridges have opposite flanks, and the decreasing of the width of the thread ridge occurs on one of the flanks of the ridge.
 7. Method according to claim 5, wherein the decreasing of the width of the thread ridge is distributed on both opposite flanks thereof.
 8. A thread-provided part produced according to the method of claim
 1. 9. A bolted joint comprising a first part having an internal thread and a second part having an external thread, and the parts are two thread-provided parts which are tightened together with each other for forming the joint, wherein at least one of the thread-provided parts is produced according to the method of claim
 1. 10. A method for producing of threads of a thread-provided part to be included as one of two thread-provided parts of a bolted joint, wherein the bolted joint comprises a first part having an internal thread and a second part having an external thread, the thread producing method comprising manufacturing the threads using a cutting tool in the threads according to a standard with appurtenant initial axial and radial play, and then axially advancing the cutting tool only an additional distance, while retaining with unaltered radial position, for achieving an additional selected axial play ΔS_(a), the additional distance corresponding to the additional axial play.
 11. A thread-provided part produced according to the method of claim
 10. 12. A bolted joint comprising two thread provided parts, with one part screwed into the other part, at least one of the thread-provided parts being produced according to the method of claim
 10. 13. The method according to claim 1, wherein the basic profile with which the threads of the respective parts are formed according to at least one of the ISO and Unified basic profile.
 14. The method according to claim 10, wherein the threads are manufactured according to at least one of the ISO and Unified basic profile. 